N-(hydrocarbyl)phosphoric or thiophosphoric triamides, their preparation and their interesting biological activity were first described in the document U.S. Pat. No. 4,530,714. They were proposed for different industrial applications, e.g. in urea-containing fertilizers to improve nitrogen utilization. There is a great demand for a process for preparing N-(hydrocarbyl)phosphoric or thiophosphoric triamides in a cost-effective and environmentally friendly way. The term “hydrocarbyl” as used herein represents alkyl or aryl groups.
N-(hydrocarbyl)phosphoric or thiophosphoric triamides are generally prepared by a two-step procedure. An intermediate N-(alkyl- or aryl)amidephosphoryl or thiophosphoryl dichloride is prepared from phosphoryl- or thiophosphoryl chloride and the corresponding hydrocarbylamine in the first step. N-(hydrocarbyl)phosphoric or thiophosphoric triamide is then prepared by the reaction of N-(alkyl- or aryl)amidephosphoryl or thiophosphoryl dichloride with ammonia in the second step. A typical representative of this group of compounds is N-(n-butyl)thiophosphoric triamide (hereinafter referred to as NBPT), the preparation of which is the object of the present invention.
U.S. Pat. No. 4,530,714 describes the preparation of N(n-butyl)amidethiophosphoryl dichloride (hereinafter abbreviated NBTDC) by the reaction of n-BuNH3Cl with 5 equivalents of thiophosphoryl chloride, 72 hours at 135° C. NBTDC is then separated from the resulting mixture by a vacuum distillation, 105-112° C., 0.75 torr. The yield of NBTDC was not mentioned there. This process is not suitable for an industrial use and thus, several improvements of this step were proposed. U.S. Pat. No. 5,770,771 describes a continuous process for the preparation of NBTDC by the reaction of thiophosphoryl chloride, n-butylamine and triethylamine, in approx. equimolar ratios, in tetrahydrofurane as the preferred solvent. The resulting mixture is a complex homogeneous phase. WO 2007/054392 A1 describes the preparation of NBTDC by the reaction of thiophosphoryl chloride, n-butylamine and low volatile tertiary amine, preferably tributylamine, in approx. equimolar ratios, in ethylacetate as the preferred solvent. The resulting mixture is a complex homogeneous phase. The above mentioned processes have serious drawbacks. There is an expert prejudice that the used inert trialkylamine exclusively binds HCl generated from the reaction in the first step. However, HCl can also react with the free n-BuNH2 present in the solution to form inert n-BuNH3Cl, which leads to a decreased conversion of phosphoryl chloride and to a longer reaction time of this step. The intermediate and all side products are in the form of a complex homogeneous phase that comprises NBTDC, the starting thiophosphoryl chloride, triethylammonium or tributylammonium chloride and their free bases, tetrahydrofurane or ethylacetate and side phospho-products. Vacuum distillation of NBTDC is necessary when a better purity of NBTDC is required.
The final product NBPT is prepared from NBTDC and ammonia in the second step. U.S. Pat. No. 5,770,771 describes a continuous process for the preparation of NBPT by the reaction of NBTDC in the form of a complex homogeneous phase obtained in the first step with a large excess of ammonia at about 0° C. in the second step. U.S. Pat. No. 5,883,297 describes a similar process for preparing NBPT starting directly from the NBTDC intermediate. WO 2007/054392 describes a method of preparing NBPT by the reaction of NBTDC in the form of a complex homogeneous phase obtained in the first step with a moderate excess of ammonia (5-6-fold excess) at 0° C. in the second step. The above mentioned processes for preparing NBPT have several drawbacks, too. The resulting NBPT prepared according to the state-of-art documents is in the form of a very complex heterogeneous and odoriferous mixture, which comprises, besides NBPT, a substantial amount of ammonium chloride and ammonia, trialkylammonium chloride and its free base, non-substituted thiophosphoric triamide, tetrahydrofurane or ethylacetate and side phospho-products. The separation of NBPT from ammonium chloride and the removal of all side odoriferous products and solvents from the complex mixture is a very tough technological and environmental problem. The purity of the resulting NBPT mentioned in the U.S. Pat. No. 5,770,771 is 92.4 and 93.4 wt. %.
U.S. Pat. No. 5,872,293 teaches the use of at least 16-fold molar excess of ammonia in the second step of the preparation of NBPT to enable the removal of the main portion of ammonium chloride from the reaction mixture in the form of a separable ammonium liquid phase. However, the proposed re-cyclation of this phase should not be repeated too many times, because of the accumulation of the impurities, thus, further treatment of this ammonium phase remains a big ecological problem. Crude NBPT is separated from the remaining liquid phase by vacuum distillation, but the product is not stable at these conditions. A stirred film vacuum evaporation is protected in U.S. Pat. No. 5,955,630 to avoid the degradation of the product. A further treatment or a method for disposal of the odoriferous distilled phase were not described there. Yields and purity of NBPT are low and further purification of NBPT is necessary. WO 2007/054392 describes an improved separation of ammonium chloride and subsequently NBPT from the complex heterogeneous phase by addition of water. The water phase with ammonium chloride is separated from the organic phase. Ethylacetate is then distilled off by vacuum distillation from the organic phase, so that low volatile tributylamine phase and product phase are formed, the product phase is then separated. Hot water is added to the product phase and NBPT is then precipitated by cooling. The improved yield of NBPT was 66% with the purity of 76 wt. %. However, such quality of NBPT is still not sufficient for an agricultural use, because the purity of NBPT of at least 85 wt. % was approved for such applications, as taught in the document “N-(n-butyl)thiophosphoric triamide”, National Industrial Chemicals Notification and Assessment Scheme (Full Public Report), Worksafe Australia, File No.: NA/467, January 1997.
Related N-(hydrocarbyl)phosphoric or thiophosphoric triamides can be prepared in a similar way to NBPT.
There are many drawbacks in preparing N-(hydrocarbyl)phosphoric or thiophosphoric triamides according to the above mentioned prior art. The separation of ammonium chloride from the product is ineffective. Large volumes of odoriferous wastes in the liquid form are economic and ecological problem. Vacuum distillation is necessary to separate the product, side products, solvent and trialkylamine from the complex heterogeneous phase. The described processes for preparing N-(hydrocarbyl)phosphoric or thiophosphoric triamides are technologically and environmentally demanding and result in low yields and low purities of the product. A repeated purification procedure is necessary to obtain the quality of the product required for an industrial use.
As follows from the above mentioned prior art, there is still a great demand for an effective and ecological process for preparing N-(hydrocarbyl)phosphoric or thiophosphoric triamides.
The technical problem underlying the present invention is therefore to provide a process for preparing N-(hydrocarbyl)phosphoric or thiophosphoric triamides with substantially improved yields and purity by a simpler and more ecological process.